Notice "as old as Moses will be when Moses will be as old as X" is just a convoluted way of saying "X".
So "Moses is twice as old as Methusaleh was when Methusaleh was one-third as old as (Moses will be when Moses is as old as Methuselah is now)" = "Moses is twice as old as Methusaleh was when Methusaleh was one-third as old as (Methusalah)"
Likewise "as old as Methusaleh was when Methusaleh was X" is just a convoluted way of saying "as old as X".
So "Moses is twice as old as (Methusaleh was when Methusaleh [was one-third as old as (Methusalah)])"= "Moses is twice as old as (one third as old as Methusalah)"
So if Moses is $A$ and Methusalah is $B$ then $A = 2*\frac 13 B$. That is all this very convoluted sentence says.
THe next sentence "The difference in their ages is 666 years" is, I hope straight forward: $|B- A| = 666$. English language, but not math, implies $B > A$ but...
So solve $A = 2*\frac 13 B$ and $|B-A| = 666$.